1. Field of the Invention
The present invention relates to a cleaning process used in a semiconductor lithographic manufacturing system and, in particular, to a method for cleaning ash reside on low dielectric constant films after photoresist removal.
2. Description of Related Art
Integrated circuits (ICs) are fabricated on semiconductor wafer substrates by a photolithographic process. The lithographic process allows for a mask pattern of the desired circuit or portion thereof to be transferred via radiant energy of selected wavelengths to a photoresist film on a substrate. Those segments of the absorbed aerial image, whose energy exceeds a threshold energy of chemical bonds in the photoactive component of the photoresist material, create a latent image in the resist. The latent image marks the volume of resist material that either is removed during the development process (in the case of positive photoresist) or remains after development (in the case of negative photoresist) to create a three-dimensional pattern in the resist film. In subsequent processing, the resulting resist film pattern is used as an etch mask to remove underlying substrates from the areas of the patterned openings in the resist layer, or to drive dopants into areas of the substrate not protected by the resist layer.
All the photoresist must be removed before the substrates are subsequently processed. Additionally, any etch-related residues must be thoroughly removed before subsequent processing to avoid embedding impurities in the device. Various methods have been disclosed for cleaning resist residue. For example, U.S. Pat. No. 6,848,455 discloses that resist contaminants are removed from a semiconductor wafer by the in-situ generation of oxidizing species, by the simultaneous application of ultra-violet radiation and chemicals containing oxidants such as hydrogen peroxide and dissolved ozone. Ultrasonic or megasonic agitation is also employed to facilitate resist removal.
Wet cleaning processes have often damaged underlying substrate layers. Plasma based chemically reactive cleaning processes have been used for traditional IC structures that employ SiO2 as the inter-layer dielectric since they do not result in any damage to the dielectric material. Such processes for stripping photoresist have employed a plasma formed from a mixture of gases with the presence of oxygen in the plasma. The highly reactive oxygen based plasma reacts with and oxidizes the organic photoresist to form volatile components that are carried away from the wafer surface.
It has been found that highly oxidizing conditions are also generally unsuitable for use on low dielectric constant (low-k) materials, i.e., those having a dielectric constant generally below about 2.7 to 3.0. Low-k materials have been used as inter-metal and/or inter-layer dielectrics between conductive interconnects employed to reduce the delay in signal propagation due to capacitive effects. The lower the dielectric constant of the dielectric material, the lower the capacitance of the dielectric and the lower the RC delay of the integrated circuit. Typically, low-k dielectrics are silicon-oxide based materials with some amount of incorporated carbon, commonly referred to as carbon doped oxide (CDO). An example of a CDO is CORAL brand carbon-doped oxides, from Novellus Systems, Inc. of San Jose, Calif. It is believed, although not necessarily proven, that the oxygen scavenges or removes carbon from the low-k materials. In many of these materials such as CDOs, the presence of carbon is instrumental in providing a low dielectric constant. Hence, to the extent that the oxygen removes carbon from these materials, it effectively increases the dielectric constant. As processes used to fabricate integrated circuits move toward smaller and smaller dimensions and requires the use of dielectric materials having lower and lower dielectric constants, it has been found that the conventional strip plasma conditions are not suitable.
Plasma processes that employ NF3 in combination with He have been used with some success for cleaning post etch residues. However, if the amount of fluorine is not controlled properly, excessive residue may remain (F ratio too low), or there is excessive loss of dielectric (F ratio too high). Excessive loss of dielectric leads to loss of critical dimension (CD), the smallest dimension of a shape, pattern or feature that can be produced by the lithographic manufacturing system.
Hydrogen plasmas or hydrogen-based plasmas with a weak oxidizing agent are effective at stripping photo-resist and removing residues from low-k dielectric layers without the problems associated with conventional strip plasmas. However, these methods require a high hydrogen flow to achieve an acceptable strip rate. Because high hydrogen flow requires costly abatement and pump systems, it is desirable to have hydrogen flow as low as possible while maintaining an acceptable strip rate. In addition, it is desirable to reduce hydrogen flow due to hydrogen's flammability and the dangers associated with handling and abating it.
Others have reported using hydrogen-based plasmas with inert gases such as hydrogen and helium introduced with hydrogen at the plasma source. Han et al. U.S. Pat. Nos. 6,281,135 and 6,638,875 describe using a mixture of hydrogen, helium and fluorine and Zhao et al. U.S. Pat. Nos. 5,660,682 and 6,204,192 describe using a mixture of hydrogen and argon. However, helium or argon ions in the plasma have harmful effects. Mixtures of hydrogen and helium result in high plasma damage on low-k materials due to the long life of ionized helium plasma. Ionized argon causes unwanted sputtering of the quartz material in the plasma tube (the portion of some reactors where the plasma is formed). Introduction of argon to hydrogen plasmas has also been shown to reduce strip rate.
U.S. application Ser. No. 11/011,273 owned by the assignee of the instant invention discloses an improved method and an apparatus for stripping photoresist and removing etch-related residues from dielectric materials. After generating a plasma from a hydrogen gas, optionally containing a weak oxidizing agent such as carbon dioxide, an inert gas is introduced to the plasma downstream of the plasma source and upstream of a showerhead that directs gas into the reaction chamber. The inert gas mixes with the plasma, reducing the required hydrogen flow rate and improving the low-k dielectric strip rate and strip rate uniformity.
Despite the improvements of the latter in the ashing or stripping of the low-k photoresist materials, it has been found that some high silicon content residue may still remain after such hydrogen plasma cleaning.
Consequently, a need exists in the art for the development of an alternative or additional stripping and cleaning process that effectively remove post-ashing reside and that does not remove excessive amounts of the low-k dielectric materials or otherwise affect or materially alter the properties of low-k dielectric materials.